1. Field of the Invention
The present invention relates to a spectroscopic apparatus which uses a varied line spacing diffraction grating.
2. Description of the Related Art
The spectroscopic apparatus which, from a non-monochromatic incident light, outputs monochromatic light having a desired wavelength has found widespread use in various physical experiments and measurements. As the spectroscopic apparatus which outputs light in the soft x-ray region, a spectroscopic apparatus using a diffraction grating (for example, a laminar-type diffraction grating having rectangular grooves) is known. In this case, with the spectroscopic apparatus using the diffraction grating, by selecting an incident angle or an exit angle (diffracted angle) with respect to the grating normal of the diffraction grating, a monochromatic ray having a desired wavelength can be obtained. Here, if a concave diffraction grating is used, image formation can be simultaneously performed in addition to spectral dispersion.
With such a diffraction grating for the soft x-ray region, the wavelength range (energy range) which can be obtained using one diffraction grating is limited, and in the range beyond this limit, the diffraction efficiency is greatly reduced or the resolution is greatly lowered. Therefore, a structure for the purpose of widening this range has been proposed. For example, in Patent Literature 1, there is disclosed a varied line spacing (VLS) diffraction grating which has widened this range by providing the lattice plane with a distribution of line spacings rather than a uniform line spacing. Further, Patent Literature 2 discloses a configuration in which the diffraction plane of the diffraction grating is divided into a plurality of regions, and in the respective regions, the range of wavelengths to be accommodated is optimized.
In case of a spectroscopic apparatus which uses a VLS concave diffraction grating for accommodating light in the soft x-ray region, the energy range in which a practicable diffraction efficiency and resolution can be maintained is 60 to 250 eV (20 to 5 nm wavelength) or so, assuming that, for example, the central value of the grating constant is set at 1/1200 mm, and the incident angle (the angle between the grating normal and the incident ray) is at 87°. Since the image plane for a diffracted light by a VLS concave grating can be made flat, in the case where a two-dimensional detector, such as a charge coupled device (CCD), is used as a detector, the light in the relevant energy region can be simultaneously detected. This is a feature of the spectroscopic apparatus using a VLS concave grating. Further, assuming that the lattice constant is set at 1/2400 mm, and the angle of incidence at 88.7°, the energy range of 250 to 1240 eV (5 to 1 nm wavelength) or so is obtained. Also in this case, since the image plane can be made flat, the light in the relevant energy region can be simultaneously detected. However, as a matter of fact, it is difficult to obtain a single VLS concave grating with both high diffraction efficiency and high resolution uniformly in the range of, for example, 60 to 1240 eV.
Therefore, in Patent Literature 3, for example, there is disclosed a technology which uses a plurality of VLS gratings by switching over among them. This technology employs translation or pivoting movement to switch over among the plurality of VLS gratings.